1. Field of the Invention
The present invention relates to a hybrid vehicle provided with an engine and a motor for driving the vehicle, and more specifically to a catalyst temperature control apparatus for warming up a catalyst that converts gaseous exhausts of the engine.
2. Description of the Related Art
A catalyst carried on an exhaust system of the engine is generally not activated at temperatures of lower than a predetermined level and can not convert the gaseous exhausts at a high efficiency. There are, however, some cases, in which the temperature of the catalyst is lower than the predetermined level; for example, under a fuel cut control or at the time of a cool start. Known techniques for enhancing the conversion efficiency of the catalyst in such cases include those disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 7-71236 and No. 6-178401. In these known techniques, a heater is placed near the catalyst and turned on to heat the catalyst when the temperature of the catalyst decreases to or below a predetermined level, thereby enabling the catalyst to maintain the high conversion efficiency of the gaseous exhausts.
The prior art techniques utilize the electric power supplied from a battery or the electric power converted from a driving force of the engine and generated by a generator, so as to actuate the heater and heat the catalyst through heat conduction. This structure, however, causes the problems of a slow supply of the electric power in some charging states of the battery and a poor response to the temperature increase.
A hybrid vehicle, on which an engine and a motor work in combination, has been proposed and expected to improve the fuel consumption rate and the emission characteristics of the gaseous exhausts. The hybrid vehicle can use the electric power stored in the battery and run only by the driving force of the motor. On the hybrid vehicle, the engine can be driven by the power smaller than the required energy for running, and a fuel cut control that temporarily cuts a supply of fuel to the engine may be carried out frequently and for a long time period. The temperature of the catalyst is often lowered to an undesirably low level in such cases. The technique for warming up the catalyst more quickly and efficiently has thus been highly demanded.